A sunscreen composition is a topical product comprising one or more UV filter compounds that absorb, scatter, and/or reflect some of the sun's ultraviolet radiation on the skin when it is exposed to sunlight. The UV filter compound(s) can be classified into three groups: chromophore-containing organic compounds such as para-aminobenzoic acid (PABA), organic particulates such as such as methylene bis-benzotriazolyl tetramethylbutylphenol (TINOSORB M by CIBA®), and inorganic particulates such as kaolin, talc, and metal oxides (e.g. zinc oxide, titanium dioxide).
Inorganic particulates are physical-barrier UV filters that reflect both UV-A and UV-B rays. Inorganic particulates offer the broadest protection against UV rays (i.e. below 290 nm to around 380 nm) that no single organic UV absorber can match. In addition, inorganic particulates provide high sun protection factors (SPFs), are economical to use, are completely stable on exposure to sunlight if properly treated, and do not cause skin problems because they are not absorbed into the skin.
As shown in this assignee's pending application Ser. Nos. 11/891,281 and 12/022,758 filed on Aug. 9, 2007 and Jan. 30, 2008, respectively, the disclosures of which are hereby incorporated by reference, it has been found that alkoxy substituted α-cyano-β, β-diphenylacrylates (alkoxycrylenes), particularly methoxycrylenes, return chromophore-containing UV filter agents, particularly butyl methoxydibenzoylmethane (Avobenzone), octyl methoxycinnamate (Octinoxate), and octyl salicylate (Octisalate), from both an electronically excited singlet state and excited triplet state back to their ground state, thereby photostabilizing the UV-absorbing organic molecules.
Chromophore-containing UV filter agents work by absorbing ultraviolet light. The absorption of ultraviolet light by a chromophore-containing organic molecule causes the excitation of an electron in the chromophore moiety from an initially occupied, low energy orbital to a higher energy, previously unoccupied orbital. The energy of an absorbed photon is used to energize an electron and cause it to “jump” to a higher energy orbital, see Turro, Modern Molecular Photochemistry, 1991. Two excited electronic states derive from the electronic orbital configuration produced by UV light absorption. In one state, the electron spins are paired (antiparallel) and in the other state the electron spins are unpaired (parallel). The state with paired spins has no resultant spin magnetic moment, but the state with unpaired spins possesses a net spin magnetic moment. A state with paired spins remains a single state in the presence of a magnetic field, and is termed a singlet state. A state with unpaired spins interacts with a magnetic field and splits into three quantized states, and is termed a triplet state.
In the electronically excited state, the chromophore-containing organic molecule is prone to degrade via a number of known pathways and, therefore, can absorb little or no additional UV light, see Bonda, Cosmetics & Toiletries 123(2):1-11, (2008). To photostabilize an electronically excited chromophore-containing organic molecule in order to provide sufficient UV protection, it must be returned to the ground state before it undergoes a photochemical reaction destructive to its UV absorbing capability. Unlike the alkoxycrylene compounds that were unexpectedly found to be capable of quenching both excited singlet state energy and excited triplet state energy, other known photostabilizing sunscreen additives, such as octocrylene, methylbenzilydene camphor, and the esters or polyesters of naphthalene dicarboxylic acid of this assignee's U.S. Pat. Nos. 6,113,931; 6,284,916; 6,518,451; and 6,551,605, all hereby incorporated by reference, are only capable of quenching excited triplet state energy.